SUMMARY Due to advances in neonatal and perinatal care, premature infants can survive at extremes of gestation (> 22 weeks). In order to sustain life following extreme prematurity, mechanical ventilation and supplemental oxygen are employed, which can disrupt normal lung development and blunt the growth of distal airspaces. This results in continued dependency on supplemental oxygen beyond 36 weeks corrected gestation, referred to as bronchopulmonary dysplasia (BPD). Chitinase 3-like-1 (Chi3l1) is a member of the evolutionary conserved 18 glycosyl hydrolase family, which is downregulated in adult mouse lungs and human bronchial epithelial cells exposed to hyperoxia, as well as in tracheal aspirates from premature infants that developed BPD. Although Sp1 transcription factor has been characterized as a strong activator of the Chi3l1 gene promoter, the mechanisms underlying the reduction of Chi3l1 by hyperoxia are not understood. Another protein whose levels are altered in hyperoxia is heme oxygenase-1 (HO-1), the rate-limiting enzyme in heme degradation. HO-1 has been shown to localize to the nucleus in neonatal mouse lungs, which can downregulate the activity of Sp1 transcription factor. Our preliminary data show that HO-1 knockout mouse embryonic fibroblasts have higher levels of Chi3l1 than wild type cells. Hence, we speculate that nuclear HO-1 could regulate Chi3l1 gene expression through modulation of Sp1. Chi3l1 has been shown to attenuate hyperoxia-induced lung apoptosis and acute lung injury in adult mice. Given lung injury responses to hyperoxic exposure differ between neonatal and adult mice, it is not known whether Chi3l1 protects against hyperoxia-induced arrest of alveolarization in neonatal mice. Autophagy has been shown to act as a protective response to reduce stress-induced apoptosis. We have preliminary data showing that mice overexpressing Chi3l1 have increased levels of autophagy marker, LC3-II in the lungs. Currently, there is no evidence indicating that Chi3l1 utilizes autophagy to mediate its protective responses to hyperoxic exposure. We hypothesize that Chi3l1 is regulated by HO-1 in basal conditions and that Chi3l1 mediates its protective response against hyperoxic lung injury through the induction of autophagy in neonatal mice. Therefore, we propose to: 1) determine the mechanisms by which HO-1 regulates the expression of Chi3l1; 2) determine how Chi31 induces autophagy during hyperoxia-induced apoptosis. This proposal will not only evaluate how Chi3l1 is transcriptionally regulated by HO-1, but also unravel novel molecular mechanisms underlying apoptotic cell death and lung injury induced by hyperoxia.